Method of production of steel sheet semi-finished products by press hardening with locally-modified structure in spots for welding

ABSTRACT

A method for producing steel sheet semi-finished products (P) by press hardening is provided that utilizes locally-modified structure in spots for welding. During this method, a steel semi-finished product (P) is heated to a temperature at which complete transformation of ferritic-pearlitic structure to austenite occurs. Afterwards, the semi-finished product (P) is cooled locally in spots for welding to a temperature close to the Mf, after which heat conduction from the surroundings causes reheating and consequently annealing of the hardening structure. Subsequently, the semi-finished product (P) is formed into a drawn part which, at the same time, is hardened in the tool, after which the sheet semi-finished product (P) is retrieved from the tool, once the required temperature was reached.

RELATED APPLICATIONS

This application claims the priority benefit of Czech Patent Application Serial No. PV 2019-513 entitled “Method of production of steel sheet semi-finished products by press hardening with locally-modified structure in spots for welding,” filed Aug. 7, 2019, the entire disclosure of which is incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to a method of producing steel sheet semi-finished products by press hardening with locally-modified structure in spots for welding.

BACKGROUND ART

Materials for manufacturing steel sheet semi-finished products may include high-strength MnB steels, which possess a martensitic microstructure after processing. Such materials may be useful in car body construction. These materials may be produced by press hardening, which is a method of hardening in a tool. Therefore, these materials may show good strength, but may also exhibit a relatively poor ability to undergo plastic deformation, which has highly adverse consequences in welded structures, namely spot-welded joints. The risk of the structure failing under external loading, arising for instance from a car collision, increases with strength. Fractures driven by low energy may occur in welds or in adjacent regions. This risk can be reduced by local annealing of the material in and/or adjacent to locations of welds. Annealing improves ductility and reduces strength values to some extent. The drawbacks to this procedure include longer production times and the energy needed for the local annealing operation. Referring to background art, Patent Application Publication No. US20190226045A1 describes a composition and method of production of a sheet of ferritic steel by annealing.

SUMMARY

One or more embodiments of the present invention generally concern a method for producing steel sheet semi-finished products by press hardening with locally-modified structure in spots for welding. Generally, the method involves heating a steel semi-finished product to a temperature at which complete transformation of ferritic-pearlitic structure to austenite occurs. Afterwards, the semi-finished product is cooled locally in spots for welding to a temperature close to the martensite finish (M_(f)), after which heat conducted from the surroundings causes the temperature in these spots to rise to a temperature, at which a hardening structure becomes tempered, while the semi-finished product is transferred to a tool, in which the sheet blank is formed into a drawn part which, at the same time, is hardened in the tool. Subsequently, the sheet semi-finished product is retrieved from the tool once the required temperature is reached.

Furthermore, in various embodiments, the method may involve heating a steel semi-finished product to a temperature within the range of 800° C. to 980° C., where the energy supplied for heating and austenitizing is used in part for tempering the hardened spots in locations of future welds.

Furthermore, in various embodiments, the method involves locally and intensively cooling a steel semi-finished product with a cooling medium in spots for welding for 1 to 10 seconds.

DISCLOSURE OF INVENTION

This invention generally relates to a method for producing steel sheet semi-finished products with a locally modified structure. More particularly, a method for producing steel sheet semi-finished products is provided that involves press hardening with locally-modified structure in spots for welding, wherein a steel blank is heated to a temperature at which ferritic-pearlitic structure completely transforms to austenite.

In various embodiments, the steel semi-finished product is initially heated to a temperature within the range of 800° C. to 980° C. or 930° C. to 960° C.

Subsequently, the semi-finished product is cooled locally, in spots to be welded, to a temperature close to the martensite finish temperature (M_(f)), i.e., only in the locations where welds are to be made. Generally, local cooling of the steel semi-finished product in spots to be welded takes between 1 and 10 seconds. Cooling may be performed during transfer to a tool.

This method of cooling produces hardened spots in the semi-finished product, whereas the temperature of the remaining material is above the temperature of transformation to hardening structures, martensite and possibly bainite. Cooling is preferably affected by means of nozzles through which pressurized cooling medium is supplied to the spots to be cooled. A shroud guides the medium away from the cooled spot. The shroud also secures that a defined spot is cooled. The intensity of the process can be enhanced by providing cooling on both faces of the sheet blank. In alternative embodiments, a contact-based cooling method may be used, which involves pressing cooled blocks against the semi-finished product surface. Cooling is ended after the spot reaches a temperature at which a hardening structure forms. Intensive cooling dissipates heat energy only from the area to which it is applied, i.e., the spot where a weld is to be made. Cooling also removes the transformation energy released during transformation of austenite to hardening structure.

Heat conducted from the surroundings of the cooled spot reheats the material in the spot, which initiates annealing of its hardening structure. Since the temperature of the blank has decreased, the cooled spots cannot reaustenitize because the temperature in their surroundings is now below the Ara temperature (i.e., the temperature when the austenite begins to cool down to reverse to become ferrite again), and, therefore, the spots cannot be hardened again in the process.

The heated blank is then placed into a tool and deep drawing is performed, at the end of which the semi-finished product rests against a mould which removes heat from the semi-finished product, thereby causing austenite to transform to martensite. The resulting temperature gradient causes some of this energy to enter the undercooled spot, where this energy contributes to tempering of fresh martensite, which gradually changes to tempered martensite or bainite, i.e., ferrite and carbides.

Having reached the martensite finish temperature, the hardened press-formed part is removed from the mould and left to cool to room temperature in air. In other embodiments, the press-formed part can be removed at a different temperature, such as room temperature or at higher temperatures, when worked in a heated tool.

DESCRIPTION OF DRAWINGS

FIG. 1 shows a sheet semi-finished product being cooled in spots where welds are to be made; and

FIG. 2 shows cooling provided on both faces of the sheet semi-finished product.

KEY TO DRAWINGS

-   1 nozzle -   2 shroud -   3 sleeve -   P steel sheet to be processed -   M flow of cooling medium -   I. Supply of cooling medium -   II. Removal of cooling medium -   III. Region under cooling

BEST MODE FOR CARRYING OUT THE INVENTION

Steel blank P made of a sheet stock of 22MnB steel of 1.5 mm thickness is placed into a continuous heating furnace. It passes through the furnace for five minutes and heats up to 950° C., at which the initial ferritic-pearlitic structure transforms to austenite. Once this heating austenitizing process ends, the semi-finished product P is pushed out from the furnace and is set, with the aid of guides and stops and after two seconds, in an exactly-defined position in the cavity of a cooling fixture. At the same time, it is approached by a cooling head fitted with cooling nozzles 1. Over 2.5 seconds, the spots to be welded are cooled with water M pressurized at 6 bar. The cooling medium M is screened from the surroundings by a concentric shroud 2 whose diameter is 12 mm. The shroud 2 also guides the cooling medium M away from the spot being cooled. After the cooling ends, the semi-finished product P is transferred to a tool by means of a handling device. 5 to 7 seconds after the end of cooling, the tool shuts and shapes the sheet semi-finished product P by plastic deformation into a drawn part, which is hardened in the tool at the same time. The semi-finished product P is then removed from the tool and cooled to room temperature.

In another preferred embodiment, the sheet semi-finished product P is removed from the furnace, placed directly in the guides of the tool, while a cooling head 1 is positioned between the top of the mould and the blank holder, and immediately after the sheet P is set inside the mould, the cooling head performs the above-described cooling by pressing annular cooling blocks against the surface of the semi-finished product. The cooling head is then withdrawn from the mould, and there is a 5 to 10 second pause followed by deep drawing and hardening in the tool. When the tool temperature reaches room temperature, the semi-finished product P is retrieved from the tool.

In another preferred embodiment, the semi-finished product P can be pushed out from the furnace and transferred by handling grippers into a tool. The grippers are fitted with the above-described nozzles, which cool the spots by supplying a cooling medium for 3 seconds in the course of the transfer. The spots are then reheated by heat entering from the surroundings by conduction. Within the next 5 seconds, the semi-finished product is positioned inside the tool with the use of guides and stops, after which the sheet semi-finished product P is shaped by plastic deformation into a drawn part and is hardened in the tool. The semi-finished product P is then removed from the tool and cooled to room temperature.

In another preferred embodiment, the grippers do not include nozzles but cooled copper blocks which, when the semi-finished product is gripped by the grippers, are pressed by the grippers against the spots in the semi-finished product and achieve the desired cooling by heat dissipation over 4 seconds. The process then proceeds in the same fashion as the previous case.

INDUSTRIAL APPLICABILITY

The invention can find broad use in the production of high-strength steel semi-finished products by means of press hardening, where improvement of mechanical properties in regions adjacent to weld joints is required. 

1. A method for producing steel sheet semi-finished products by press hardening with locally-modified structure in spots for welding comprising: (a) heating a steel semi-finished product to a first temperature at which complete transformation of ferritic-pearlitic structure to austenite occurs to thereby form a semi-finished product; (b) locally cooling the semi-finished product in spots for welding to a second temperature close to the martensite finish temperature, wherein generated heat causes temperature in the spots to rise to a third temperature at which a hardening structure becomes tempered; and (c) transferring the semi-finished product to a tool, in which a sheet blank is formed into a drawn part which, at the same time, is hardened in the tool, after which a sheet semi-finished product is retrieved from the tool once a required temperature is reached.
 2. The method according to claim 1, wherein the semi-finished product is heated to a temperature within the range of 800° C. to 980° C., wherein the energy supplied for heating and austenitizing is used in part for tempering hardened spots in locations of future welds.
 3. The method according to claim 1, wherein the semi-finished product is locally and intensively cooled by a cooling medium in the spots for welding for 1 to 10 seconds. 